The two words, "coherence" and "cohesion" mean different
things but the two ideas are connected and, in fact, overlap. "Coherence" in an
essay (or when you are speaking about interconnected idea in the Speaking test)
means the overall "understandability" of what you write or say. When writing an
essay, coherence involves such features as: summarizing the overall argument of
an essay in the introductory paragraph; presenting ideas in a logical sequence;
putting separate, major points into separate paragraphs; and beginning each
paragraph with a 'topic sentence', following by supporting sentences. Coherence
is based more on the logic of the ideas and how they are presented rather than
on the language that is used to express these ideas.

"Cohesion" refers to the degree to which sentences (or even
different parts of one sentence) are connected so that the flow of ideas
is easy to follow. To achieve good cohesion, you need to know how to use
"cohesive devices", which are certain words or phrases that serve the purpose of
connecting two statements, usually by referring back to what you have previously
written or said. For example, if you write "Statement A" and then follow with
the words, "On the other hand,
Statement B", then these two sentences "cohere" or "stick together" and it is
easy to follow the flow of ideas. Good cohesion leads to good coherence, which
is the ultimate aim.

Here's an example of how coherence and cohesion overlap. If
you have a major new point to add to your essay then you should put that in a
paragraph by itself and begin the paragraph with a topic sentence that more or
less summarizes the point you want to make. This topic sentence, following by
supporting sentences, make your paragraph more coherent. However, your essay will be
less coherent if you suddenly start a paragraph without some form of connection
to what you have previously written, either in the previous paragraph or some
other previous part of your essay. This problem can be overcome by beginning that topic
sentence with words such as, "On the other hand", which connect to the last
statement made in the previous paragraph. This shows good cohesion.

For both Task 1 and Task 2 of the Writing test, "Coherence
and Cohesion" is one of the four items that is given a sub-score. The other
three items are: ‘Task Response’ (or, ‘Task Achievement’ for Task
1), ‘Lexical Resource’ ( = vocabulary) and ‘Grammatical Range and
Accuracy’.